PROJECT SUMMARY/ABSTRACT Circadian rhythms are the near-24-hour rhythms of physiology ubiquitous to almost all eukaryotic life. Dysfunction of circadian rhythms underlies a variety of common sleep disorders and is thought to contribute to other conditions ranging from psychiatric disease to cancer. The mammalian retina serves a critical function in synchronizing the master circadian pacemaker (the suprachiasmatic nucleus) to the daily light-dark cycle. Work over many years has also demonstrated that the retina itself is a strong circadian oscillator. Indeed, many critical retinal functions, including visual sensitivity, the pupillary light reflex, the electroretinogram, and the expression of hundreds of retinal genes, are under strong circadian control; and that loss of retinal circadian rhythms results in impaired retinal function. Our preliminary data have demonstrated that: 1) the retinal circadian clock can be entrained to light- dark cycles in culture ex vivo, 2) this entrainment is not dependent on the classical rods and cones or the melanopsin-expressing, intrinsically-photosensitive retinal ganglion cells, 3) the orphan opsin neuropsin (OPN5) is necessary for this photoentrainment, and the orphan opsin encephalopsin (OPN3) affects this process, 4) the retina utilizes a light-dependent, diffusible substance to synchronize its rhythms, and 5) the cornea also contains a circadian clock which, remarkably, can be entrained to light- dark cycles as well via an OPN5-dependent mechanism. We propose experiments to elucidate the signaling mechanisms of OPN5 and OPN3; experiments to characterize the diffusible signal(s) emanating from the retina, and experiments to elucidate the mechanism by which non-retinal tissues in the eye maintain circadian rhythmicity and entrain to light-dark cycles. These data will provide a critical basis for understanding how the circadian clock modulates retinal function as well as mechanistic insights into two novel ocular photoreceptors.